1. Field of the Invention
The present invention relates to an optical pickup apparatus which detects a focusing error signal by the "knife edge" method.
2. Description of the Related Art
In general, an optical pickup apparatus requires a tracking servo control unit and focusing servo control unit in order to focus a beam on an optical recording media, and follow a beam on the track when recording, reproducing and erasing information.
The present invention corrects the focusing error by detecting a focusing error signal and controlling a focusing servo, in response to said detection. The asymmetry aberration method or "knife edge" method is used as the method for detecting the focusing error signal.
Furthermore, the present invention corrects tracking error by detecting a tracking error signal for controlling the tracking servo. There are many kinds of methods for detecting such a tracking error signal. One of these methods is the "three beam" method which uses a main beam and two sub-beams. The sub-beams are formed before and behind the main beam in the direction of the track. This method is disclosed in detail in "Optical Pickup System Point of Development" which is published by the Japan Industrial Technique Center. When using the "three beam" method, the tracking error signal is detected as a difference in the amount of optical information between the two sub-beams. Therefore, it is customary to shift the sub-beams to positions one fourth of the track pitch from the track center in a direction across the track for getting the highest detective sensitivity. In other words, the sub-beams appear at positions one fourth of the track pitch before and after the main beam.
Another method for detecting the tracking error signal is a "differential push-pull" method. A main beam and two sub-beams are also used in this method. However, the positions of the sub-beam spots are shifted half track pitch(as opposed to one fourth track pitch). A tracking error signal is detected by detecting the push-pull signals of the reflective rays of the main beam and sub-beams respectively, and obtaining the difference between them.
However, when the "three beam" method or the "differential push-pull" method is applied to a pickup apparatus detecting the focusing signal by the "knife edge" method, the shading rate for each reflective beam is different. One sub-beam is mostly reflected by the knife edge and the other is inclined not to be reflected. Therefore, there is a difference in the shading rate for the two sub-beams. This causes the amount of optical information received by a photo detector to be unequal. Therefore, even if there is not tracking error, a tracking error is registered due to the inequality in the shading rate.